4.1 (8) - DNA, Genes and Protein Synthesis Flashcards
Genes and the triplet code DNA and chromosomes The structure of RNA Protein synthesis - transcription and splicing Protein synthesis - translation
What 3 things is a nucleotide made from?
- Phosphate group
- Deoxyribose sugar (pentose)
- Nitrogenous, organic base
What are the base pairing rules?
A + T (held together by 2 hydrogen bonds)
G + C (held together by 3 hydrogen bonds)
What is DNA?
DNA is a hereditary, information-carrying molecule
What is the minimum number of bases needed to code for an amino acid? Why?
3, because pairs wouldn’t be able to produce the range of amino acids we need
What are some features of the genetic code?
- Degenerate code
- Triplets always read in the same way
- Have start and stop codes in DNA sequencing
- Each sequence is only read once
- Universal code
What is a degenerate code?
When most amino acids are coded for by more than one triplet (eg. an amino acid could be made from ATT or AAG).
What is a universal code?
When each triplet codes for the same amino acid in all organisms
Which direction does the left strand run in?
5’ to 3’
Which direction does the right strand run in?
3’ to 5’
What are some functions of antiparallel strands?
- Maintains 2 copies of the DNA, in case of mutation
- Protects bases from reacting with other bases
- Required for semi-conservative replication
- Improves molecular stability
What is a gene?
- A unit of heredity
- A section of DNA on homologous chromosomes coding for one or more polypeptides
What is DNA like in a eukaryotic cell?
- DNA in a double membrane-bound nucleus
- Wound round histone proteins, to condense it
- Lots of linear chromosomes
- No plasmids
- Has a lot of introns (DNA that don’t code for polypeptides)
- Has exons (coding sequences within genes)
- Some genes code for rRNA and tRNA
What is DNA like in a prokaryotic cell?
- DNA is free in the cytoplasm
- Naked
- Single circular chromosome
- May contain plasmids (smaller loops of DNA)
What is a homologous chromosome?
2 chromosomes (1 maternal and 1 paternal) that have the same loci, which allows them to determine the same features
What is a human’s diploid number?
46 (23 homologous chromosomes)
What is a locus?
The specific point on a chromosome where a gene is located.
What is a genome?
The complete set of genes in a cell
What is a proteome?
The full range of proteins produced by a genome
Give 4 ways in which RNA is different to DNA
- RNA is made with ribose nucleotides, DNA is made with deoxyribose nucleotides
- RNA has the base ‘uracil’ instead of ‘thymine’
- RNA is single-stranded, DNA is double-stranded
- RNA is shorted than DNA
What is the molecular difference between deoxyribose and ribose?
On the 2’ carbon, ribose has a hydroxyl group
Deoxyribose doesn’t have this
What are the 3 types of RNA?
mRNA, tRNA and rRNA
What is the function of RNA?
It transfers coded information from DNA into the cytoplasm where is can be translated into proteins.
What is the function of mRNA?
- The code for a gene in DNA is transcribed (copied) into an mRNA molecule.
- mRNA carries the code for a particular polypeptide from the nucleus to the cytoplasm. It can travel through nuclear pores.
What is the function of tRNA?
tRNA matches the codons in an mRNA molecule to their constituent amino acids
Describe a tRNA molecule
- On the middle loop, there is a triplet nucleotide sequence called an anticodon, which is complementary to one of the 64 possible codons on the mRNA.
- Each amino acid is attached to a tRNA with the appropriate anticodon.
What is the function of rRNA?
Together with proteins, it forms ribosomes
What are codons?
Triplets of base pairs on a mRNA molecule.
What is transcription?
The process of making pre-mRNA using part of the DNA as a template
If a coding DNA sequence is CGA, what is the pre-mRNA sequence?
GCU
If a pre-mRNA sequence is UCA, what is the coding DNA sequence?
AGU
What is a DNA template (coding) strand?
A DNA template strand is the strand used by DNA polymerase to attach complementary bases to during DNA replication
Describe the 6 stages of transcription
1) DNA helicase breaks the hydrogen bonds on the DNA molecule, exposing its bases.
2) RNA polymerase binds to a section of DNA.
3) The nucleotides on 1 of the 2 DNA strands (template strands) bind to complementary mRNA nucleotides present in the nuclei.
4) RNA polymerase moves along the strand, joining the nucleotides together to make a pre-mRNA molecule.
5) While mRNA nucleotides are attaching, the DNA strands re-join behind the newly-forming strand of pre-mRNA.
6) When the RNA polymerase reaches the stop codon (end of the triplet code), it detached and the pre-mRNA is complete.
Explain and describe the process of splicing pre-mRNA
1) The non-functioning introns are later removed from the pre-mRNA sequence.
2) The remaining exons are joined together by splicing to form mature mRNA.
3) The exon sections can be joined together in a variety of combinations and orders, so the same sequence of DNA can code for several different proteins
Why doesn’t the splicing of pre-mRNA happen in prokaryotes?
They don’t have introns, so it isn’t necessary.
What is translation?
Translation is the process of converting the codon sequence in the mRNA molecule into an amino acid sequence
Describe the 7 stages of translation
1) mRNA molecule leaves the nucleus via nuclear pores after splicing
2) A ribosome attaches to the start codon of the mRNA
3) A tRNA molecule, with the complementary anticodon sequence to the first mRNA codon, pairs with the codon on the mRNA
4) The ribosome has 2 codon binding sites, so 2 tRNA molecules at a time are brought together
5) A peptide bond forms between the 2 amino acids on the tRNA with the use of an enzyme and ATP (ATP hydrolysed into ADP and Pi to provide energy)
6) The ribosome moves along the mRNA to the next codon, so the first tRNA is released and a new tRNA arrives at the next codon
7) This continues, with up to 15 amino acids being added every 15 seconds, until a polypeptide chain is built
